Solid-state image pickup device, having increased charge storage and improved electronic shutter operation

ABSTRACT

The impurity density of a photoelectric transducer n-layer (7) and the impurity density of a p-layer (6) of an impurity region in which the electric transducer (7) and a transfer channel (9) are formed, are each distributed to have its maximum value in a more interior part from the surface of a semiconductor substrate (5). Alternatively, i) a thin, high-density p-layer (34) and ii) a thick, low-density p-layer (33) of an impurity region in which the electric transducer (7) and the transfer channel (9) are formed may be formed. Each minimum potential in these two p-layers (33, 34) is made to have a different dependence on the voltage applied to an n-type semiconductor substrate (5). The thick, low-density p-layer (33) is formed in such a way that it comes into contact with part of the photoelectric transducer n-layer (7) at its bottom portion. The above constitution can bring about a solid-state image pickup device that can prevent the blooming phenomenon, causes less residual images, and can operate as an electronic shutter with ease.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a solid-state image pickup device, a processfor manufacturing the device and a method of driving the device.

2. Description of the Prior Art

In recent years, solid-state image pickup devices making use of chargetransfer systems as typified by a charge-coupled device (hereinafter"CCD") have been markedly put into practical use in view of theirlow-noise characteristics.

A structure commonly used in a conventional solid-state image pickupdevice will be described below with reference to accompanying drawings.

FIG. 24 is a plan view of what is called a CCD type solid-state imagepickup device. The solid-state image pickup device is comprised of aphotoelectric transducer area 1, a vertical CCD transfer electrode area2, a horizontal CCD transfer electrode area 3 and an output area 4.

Upon incidence of light emitted from an object on the photoelectrictransducer area 1, electron-hole pairs are produced in the photoelectrictransducer area 1 by the action of light. The electrons thus producedare sent from the photoelectric transducer area 1 to the vertical CCDtransfer electrode area 2. Into the vertical CCD transfer electrode area2, electrons of photoelectric transducer areas 1 arranged in thelongitudinal direction of the vertical CCD transfer electrode area 2 aresent at the same time.

Then the electrons taken out to the vertical CCD transfer electrode area2 are sent to the horizontal CCD transfer electrode area 3. Into thehorizontal CCD transfer electrode area 3, electrons of vertical CCDtransfer electrode areas 2 arranged perpendicularly to the longitudinaldirection of the horizontal CCD transfer electrode area 3 are sent atthe same time.

The electrons thus taken out to the horizontal CCD transfer electrodearea 3 are outputted through the output area 4. Output signals thusobtained pass through an image reproducing circuit and are outputted asan image of the object from an output medium such as a display device.

FIG. 25 show a cross section of the solid-state image pickup devicealong the line 25--25 (in FIG. 24) that passes across the photoelectrictransducer areas 1 and the vertical CCD transfer electrode areas 2.

In the upper layer portion or main surface of an n-type substrate 5, ap-layer 6 having depth and density in given ranges is formed. Aphotoelectric transducer, an n-layer 7, is formed in a given region inthe interior of the p-layer 6. A p-layer 8 with a high density is alsoformed in the p-layer 6 at a position spaced apart from the n-layer 7.An n-layer 9 that serves as a vertical CCD transfer channel is providedin the p-layer 8.

Signal charges accumulated in the n-layer 7 are read out to the n-layer9 and then the signal charges are transferred within the n-layer 9 andin the direction perpendicular to the paper face of the drawing.

A high-density p-layer 10 formed on the n-layer 7 is formed so that anydark current caused by an interfacial level of Si-SiO₂ can be preventedfrom occurring.

Across the p-layer 6 and a lower layer portion 5a (or a region wherenone of p-layers and n-layers are formed; hereinafter "substrate 5a") ofthe substrate 5, a reverse bias voltage Vsub that gives positivepotential to the substrate and gives negative potential to the p-layer 6is applied by means of an electric source 11. That is to say, thereverse bias voltage Vsub is applied across the p-layer 8 and thesubstrate 5a. Hence, the p-layer 6 located below the photoelectrictransducer formed of the n-layer 7 becomes depleted. As a result ofdepletion of the p-layer 6, charges that have turned excessive withrespect to the quantity of the charges that can be accumulated in then-layer 7 are released to the substrate 5a side. It is designed toprevent the blooming phenomenon in this way.

Application of a pulse voltage to the n-type substrate 5 makes it alsopossible to release all the signal charges in the n-layer 7 to thesubstrate 5a side, that is to achieve the operation of what is called anelectronic shutter.

An insulating film 12 comprising SiO₂ or the like is formed on thesurface of the substrate 5. On the insulating film 12, a vertical CCDtransfer electrode 13 is formed above the p-layer 8 and n-layer 9 thatconstitute the vertical CCD transfer electrode area 2 and also above theregion embracing a gap region between the n-layer 7 and the p-layer 8.The insulating film 12 is also formed on the side wall and upper surfaceof the transfer electrode 13 for the purpose of protecting thesolid-state image pickup device from a physical impact or shock.

The vertical CCD transfer electrode 13 acts as an electrode for makingthe electrons accumulated in the n-layer 7 read out to the p-layer 8 orn-layer 9 that forms or serves as the vertical CCD transfer channel.

Thus, in order to prevent the blooming phenomenon or achieve theoperation as an electronic shutter, the p-layer 6 must have density anddepth in given ranges.

FIG. 26 shows impurity density distribution examined along the line26--26 passing across the photoelectric transducer area 1 in FIG. 25.

FIG. 27 shows net values of the impurity densities shown in FIG. 26.

In both FIGS. 26 and 27, the impurity densities are plotted as ordinate,and the distance from the substrate surface is plotted as abscissa. Thedensities of p-type impurity atoms are indicated on the upper side ofthe ordinate and the densities of n-type impurity atoms are indicated onthe lower side of the ordinate. Broken lines 14, 15, 16 and 17 in FIG.26 represent the impurity densities of the p-layer 10, p-layer 6,substrate 5a and n-layer 7 in FIG. 25, respectively.

A solid line 18 in FIG. 27 represents the net values of the respectiveimpurity densities shown in FIG. 26. The shaded region 19 corresponds tothe net impurity density distribution of the photoelectric transducern-layer 7, showing the amount of effective donors accumulated in then-layer.

The impurity density 17 of the photoelectric transducer n-layer 7 ishighest at the surface of the substrate 5. The density decreases with adepth toward the interior of the substrate 5. The p-layer 10 is formedat the upper part of the n-layer 7 so that the dark current can bedecreased. The impurity density 14 of the p-layer 10 is higher than thedensity of the n-layer 7, and is less spread toward the interior of thesubstrate 5. Hence, the n-layer 7 comes to have the net impurity density19.

The p-layer 6 has a low impurity density, but its impurity density isdistributed in a broadly spread state. More specifically, its impuritydensity is at maximum at the surface of the substrate 5 and graduallydecreases with a depth toward the interior of the substrate 5.

In the solid-state image pickup device, in order not to cause theblooming phenomenon, it is necessary for the photoelectric transducern-layer 7 to be almost completely depleted after the electronsaccumulated in the photoelectric transducer have been read out to thevertical CCD transfer electrode area. For this reason, the amount of theeffective donors represented by the net impurity density 19 must becontrolled to be an upper limit of the quantity of the charges that canbe accumulated in the n-layer 7. In other words, it follows that theamount of effective donors determines an upper limit value of saturationcharacteristics of the photoelectric transducer.

The net impurity density 19 is a value obtained by subtracting theimpurity density 14 of the p-layer 10 and the impurity density 15 of thep-layer 6 from a value obtained by adding the impurity density 16 of thesubstrate 5a to the impurity density 17. Because of the restrictions inview of each process, the net impurity density 19 depends on theimpurity density 14 and impurity density 17. As to the impurity density17, the density is highest at the surface of the substrate 5. The partwhere this density is highest is compensated by the impurity density 14of the reverse conductivity type p-layer 10.

FIGS. 28 to 31 cross-sectionally illustrate a flow sheet for fabricatinga conventional CCD type solid-state image pickup device.

In the main surface (i.e., the upper layer portion) of the n-typesubstrate 5, p-type impurity boron is ion-implanted. Thereafter, thep-layer 6 is formed by a high-temperature beat treatment. Thereafter, aresist pattern is formed by conventional photolithography on the regionother than the region in which the vertical CCD transfer channel isformed. Using the resist pattern as a mask, boron is ion-implanted toform the p-layer 8. Thereafter, a resist pattern is again formed byconventional photolithography on the region other than the region inwhich the n-layer 9 in the p-layer 8 serving as the vertical CCDtransfer channel is formed. Using the resist pattern as a mask,phosphorus is ion-implanted 2 to form the n-layer 9. Thereafter, theinsulating film 12 is deposited on the main surface of the substrate 5by thermal oxidation. An electrode material serving as the vertical CCDtransfer electrode 13 is further formed on the insulating film 12 (FIG.28).

Then, a resist pattern 20 is formed on the electrode material byphotolithography on the region other than the region broader than theregion that serves for the transfer electrode 13. Using the resistpattern as a mask, the electrode material is dry-etched until theinsulating film 12 is uncovered. Next, using the resist pattern 20 as amask and also making the insulating film 12 serve as a protective film,phosphorus is ion-implanted. The n-layer 7 serving as the photoelectrictransducer is formed as a result of this ion implantation (FIGS. 29 to30).

Thereafter, the resist pattern 20 is removed. On the surface of thesubstrate 5, an electrode material is formed with a region broader thanthe insulating film 12 and transfer electrode 13. Next, a resist patternis formed on the region other than the region in which the transferelectrode 13 is formed. Using this resist pattern as a mask, theelectrode material is dry-etched. Through the above process, thetransfer electrode 13 is formed. At this time, the transfer electrode 13must be provided also on the gap region between the n-layer 7, i.e., thephotoelectric transducer from which electrons are read out to thevertical CCD transfer channel, and the p-layer 8. For this purpose,since in the dry etching carried out when the transfer electrode 13 isformed a side wall end of the n-layer 7 serving as the photoelectrictransducer is originally on the same line with a side wall end of thetransfer electrode 13, one side wall end of the transfer electrode 13 isshortened to be formed in the desired length (FIG. 30).

Next, using one end of this transfer electrode 13 and a resist as masks,boron is ion-implanted. The p-layer for preventing dark current is thusformed on the n-layer 7 (FIG. 31).

The conventional solid-state image pickup device as described above hasthe following disadvantages.

The impurity density 17 is highest at the surface of the substrate 5,and this part where the density is highest is compensated by theimpurity density 14 of the reverse conductivity type p-layer 10. Hence,of the impurity density 17 of the n-layer 7 introduced as aphotoelectric transducer, the region having a relatively low density isused as the photoelectric transducer. In other words, a photoelectrictransducer with a low density results in a small amount of the effectivedonors that can be accumulated there, and can not achieve the quantityof saturation charges at a sufficiently high level.

Accordingly, the impurities to be lead into the region of impuritydensity 17 are implanted in an increased quantity. The implantedimpurities are also diffused into the depth of the substrate by ahigh-temperature heat treatment so that the photoelectric transducer canhave an increased area.

The high-temperature heat treatment, however, may result in a spread ofthe impurity density 17 in the direction perpendicular to the surface ofthe substrate 5. As a result, the impurities are also diffused in thegap region between the p-layer 8 and n-layer 7 and also in the p-layer8, the vertical CCD transfer channel.

The positional relationship between an end portion of the n-layer 7 andthe vertical CCD transfer electrode 13 is determined by the precisionfor the registration of the mask in the step of exposure carried outwhen the transfer electrode 13 is formed and also on the diffused regionof the n-layer 7 formed by diffusion. For this reason, it is verydifficult to control the position at which the n-layer 7 is formed byheat diffusion at a high temperature. A poor controllability for thepositions of the end portion of the n-layer 7 and the vertical CCDtransfer electrode 13 causes the electrons serving as signals to be leftin the photoelectric transducer at the time of read-out where they aretaken out from the photoelectric transducer to the vertical CCD transferchannel.

The electrons thus having been left cause the phenomenon of phantom orresidual images, resulting in a serious deterioration of picturequality.

On the other hand, when the impurities are implanted in an increasedquantity, faults in ion implantation increase to increase what is calledwhite scratches. This results in a lowering of the yield.

Moreover, the p-layer 6 is formed as a single impurity layer. Hence thevoltage applied to the p-layer 6 must be controlled in order to preventthe blooming phenomenon or achieve the operation as an electronicshutter.

In the device with such structure, the voltage applied to the n-typesubstrate 5 is usually controlled to be about 10 V in order to controlthe blooming phenomenon. A pulse voltage of about 30 V is also requiredin order to achieve the operation as an electronic shutter.

For the purpose of applying such voltages, the number of parts must beincreased, resulting in an increase in power consumption, whencamera-combined VTRs usually making use of the solid-state image pickupdevice are manufactured. This obstructs the manufacture of those beingsmall-sized, lightweight and also of low power consumption.

SUMMARY OF THE INVENTION

An Object of the present invention is to provide a solid-state imagepickup device that may hardly cause the blooming phenomenon, can preventdeterioration of picture quality and can achieve the operation as anelectronic shutter at a low voltage.

Another object of the present invention is to provide a process forfabricating a solid-state image pickup device, that can form the abovesolid-state image pickup device through a simple process.

In an embodiment of the solid-state image pickup device, the presentinvention provides a solid-state image pickup device comprising asemiconductor substrate; a first semiconductor region of oneconductivity type provided in said semiconductor substrate; a secondsemiconductor region of a conductivity type reverse to said firstsemiconductor region, formed in said first semiconductor region: a thirdsemiconductor region of a conductivity type reverse to said secondsemiconductor region, formed adjoiningly to the surface of said secondsemiconductor region; and a fourth semiconductor region of the sameconductivity type as said second semiconductor region, formed apart fromsaid second semiconductor region within said first semiconductor region;

said second semiconductor region having an impurity density whosedistribution in the depth direction of said semiconductor substrate hasa maximum part at a more interior part from the surface of saidsemiconductor substrate.

In another embodiment of the solid-state image pickup device, thepresent invention provides a solid-state image pickup device comprisinga semiconductor substrate; a first semiconductor region of oneconductivity type provided in said semiconductor substrate; a secondsemiconductor region of a conductivity type reverse to said firstsemiconductor region, formed in said first semiconductor region; a thirdsemiconductor region of a conductivity type reverse to said secondsemiconductor region, formed adjoiningly to the surface of said secondsemiconductor region; and a fourth semiconductor region of the sameconductivity type as said second semiconductor region, formed apart fromsaid second semiconductor region within said first semiconductor region;

said first semiconductor region and said second semiconductor regioneach having an impurity density whose distribution in the depthdirection of said semiconductor substrate has a maximum part at a moreinterior part from the surface of said semiconductor substrate.

In still another embodiment of solid-state image pickup device, thepresent invention provides a solid-state image pickup device comprisinga semiconductor substrate; a first semiconductor region of oneconductivity type, formed in a given region of said semiconductorsubstrate; a second semiconductor region of a conductivity type reverseto said first semiconductor region, formed in said first semiconductorregion and in the region between said first semiconductor region andanother first semiconductor region adjacent thereto; a thirdsemiconductor region of a conductivity type reverse to said secondsemiconductor region, formed at least on the surface of said firstsemiconductor region; a fourth semiconductor region of the sameconductivity type as said second semiconductor region, formed apart fromsaid second semiconductor region within said first semiconductor region;and a fifth semiconductor region of the same conductivity type as saidsecond semiconductor substrate, formed between said first semiconductorregion and another first semiconductor region-adjacent thereto.

In an embodiment of the process for fabricating a solid-state imagepickup device, the present invention provides a process formanufacturing a solid-state image pickup device, comprising the steps offorming a first semiconductor region on a semiconductor substrate byfirst ion implantation, forming a transfer channel in said firstsemiconductor region, forming an insulating film on said semiconductorsubstrate, forming an electrode material on said insulating film,forming a second semiconductor region by second ion implantation usingat least said electrode material as a mask, forming a transfer electrodeby etching said electrode material, and forming a third semiconductorregion adjoiningly to the surface of said second semiconductor region;

at least one of said first ion implantation and said second ionimplantation being carried out at an accelerating voltage of not lessthan 200 keV.

The present invention also provides a method of driving a solid-stateimage pickup device, said device comprising a semiconductor substrate; afirst semiconductor region of one conductivity type, formed in a givenregion of said semiconductor substrate; a second semiconductor region ofa conductivity type reverse to said first semiconductor region, formedin said first semiconductor region and in the region between said firstsemiconductor region and another first semiconductor region adjacentthereto; a third semiconductor region of a conductivity type reverse tosaid second semiconductor region, formed at least on the surface of saidfirst semiconductor region; a fourth semiconductor region of the sameconductivity type as said second semiconductor region, formed apart fromsaid second semiconductor region within said first semiconductor region;and a fifth semiconductor region of the same conductivity type as saidsecond semiconductor substrate, formed between said first semiconductorregion and another first semiconductor region adjacent thereto;

said method comprising applying a reverse bias voltage across said firstsemiconductor region and said semiconductor substrate in such a way thata period during which said reverse bias voltage is applied is comprisedof a first application period and a second application period, theminimum potential of said first semiconductor region during the firstapplication period is higher than the minimum potential of said fifthsemiconductor region and the minimum potential of said fifthsemiconductor region during the second application period is higher thanthe minimum potential of said first semiconductor region.

In the solid-state image pickup device according to the presentinvention, the-n-layer that serves as a photoelectric transducer isformed by ion implantation using a high accelerating energy, and hencethe impurity density distribution can be made to have a highest densityregion in the interior of the substrate.

This brings about an increase in the amount of the effective donors thatcan be accumulated in the photoelectric transducer, and achieves thequantity of saturation charges at a sufficiently high level.

Since the photoelectric transducer is formed by ion implantation, thepositional relationship between the end portion of the photoelectrictransducer and the vertical CCD transfer electrode can be determined bythe precision for the registration of a mask in the step of exposurecarried out when the transfer electrode 13 is formed. Hence it isunnecessary to control the position at which the photoelectrictransducer is formed by heat diffusion at a high temperature. Thus, ahigh controllability can be achieved for the positions of the endportion of the photoelectric transducer and the vertical CCD transferelectrode. This makes the solid-state image pickup device hardly causethe blooming phenomenon, and makes it possible to prevent deteriorationof picture quality.

In the impurity density distribution of the n-layer in which thephotoelectric transducer is formed by ion implantation it is possiblefor the maximum value of its density to be present in the interior ofthe substrate. This brings about an increase in the amount of theeffective donors accumulated in the photoelectric transducer. Hence, itis possible to obtain a solid-state image pickup device equipped with aphotoelectric transducer having high saturation characteristics.

The p-layer for preventing dark currents is also formed on thephotoelectric transducer n-layer. The n-layer serving as thephotoelectric transducer is formed by ion implantation to have a maximumvalue in its impurity density in the interior of the substrate. Hence,the net value of the impurity density of the p-layer at the surface ofthe substrate can be made to have a higher density than the impuritydensity of the p-layer of the conventional solid-state image pickupdevice. This makes it possible to obtain a solid-state image pickupdevice equipped with a photoelectric transducer having low dark currentcharacteristics.

Moreover, a solid-state image pickup device that can operate as anelectronic shutter at a low voltage can be formed without decreasing thequantity of the charges accumulated in the photoelectric transducer.

In the process for fabricating the solid-state image pickup device ofthe present invention, the impurity density of the photoelectrictransducer is highest in the depth of the substrate. Hence, it isunnecessary to increase the quantity of the impurities implanted in thephotoelectric transducer, which has been done in the prior art. Toincrease the quantity of impurities necessarily brings about aconsiderable loss of time. Thus, in the process of the presentinvention, there can be no lowering of throughput at the time of thefabrication of solid-state image pickup devices. In addition, it ispossible to prevent the yield from being lowered because of the whitescratches.

It is also unnecessary to diffuse the implanted impurities into thedepth of the substrate by a high-temperature heat treatment so that thearea of the photoelectric transducer can be increased. Hence, no defectscan occur in the substrate, which may be caused by the high-temperatureheat treatment. Since the impurities are diffused from other diffusionlayer, it is still also unnecessary to make control so as to attain thedesired impurity density. There also is no possibility that thephotoelectric transducer is diffused and spread as a result of thehigh-temperature heat treatment and consequently diffused also to thevertical CCD transfer channel or the gap region between the transferchannel and the photoelectric transducer electrode 13.

As for the positional relationship between the end portion of thephotoelectric transducer and the end portion of the vertical CCDtransfer electrode, it can not become difficult to control the positionof the diffusion layer as a result of the high-temperature heattreatment. Hence the positional relationship between the both can beattained with ease. This can prevent the phenomena of blooming andresidual image from occurring in the solid-state image pickup device,and also can stop the deterioration of picture quality.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section to illustrate a solid-state image pickupdevice according to a first embodiment of the present invention.

FIG. 2 is a graph to show impurity densities of the solid-state imagepickup device according to the first embodiment of the presentinvention.

FIG. 3 is a graph to show net impurity densities of the solid-stateimage pickup device according to the first embodiment of the presentinvention.

FIG. 4 is a graph to show net impurity densities of the solid-stateimage pickup device according to the first embodiment of the presentinvention.

FIG. 5 is a graph to show impurity densities of a solid-state imagepickup device according to a second embodiment of the present invention.

FIG. 6 is a graph to show net impurity densities of the solid-stateimage pickup device according to the second embodiment of the presentinvention.

FIG. 7 is a cross section to illustrate a solid-state image pickupdevice according to a third embodiment of the present invention.

FIG. 8 is a graph of potential distribution to show a bloomingphenomenon in the third embodiment of the present invention.

FIG. 9 is a graph of potential distribution to show the operation as anelectronic shutter in the third embodiment of the present invention.

FIG. 10 is a graph to show the relationship between substrate potentialand reverse bias voltage in the third embodiment of the presentinvention.

FIG. 11 is a graph to show a maximum quantity of accumulated charges inthe third embodiment of the present invention.

FIG. 12 is a cross section to illustrate a solid-state image pickupdevice according to a fourth embodiment of the present invention.

FIG. 13 is another cross section to illustrate the solid-state imagepickup device according to the fourth embodiment of the presentinvention.

FIG. 14 is a graph to show impurity densities of the solid-state imagepickup device according to the fourth embodiment of the presentinvention.

FIG. 15 is a graph to show net impurity densities of the solid-stateimage pickup device according to the fourth embodiment of the presentinvention.

FIGS. 16 to 19 show a flow sheet to illustrate a first embodiment of theprocess for fabricating the solid-state image pickup device of thepresent invention.

FIGS. 20 to 23 show a flow sheet to illustrate a second embodiment ofthe process for fabricating the solid-state image pickup device of thepresent invention.

FIG. 24 is a plan view to illustrate a conventional solid-state imagepickup device.

FIG. 25 is a cross section to illustrate the conventional solid-stateimage pickup device.

FIG. 26 is a graph to show impurity densities of the conventionalsolid-state image pickup device.

FIG. 27 is a graph to show net impurity densities of the conventionalsolid-state image pickup device.

FIGS. 28 to 31 show a flow sheet to illustrate a process for fabricatingthe conventional solid-state image pickup device.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Preferred embodiments of the present invention will be described belowwith reference to the accompanying drawings.

FIG. 1 cross-sectionally illustrates a solid-state image pickup deviceaccording to a first embodiment of the present invention.

The device shown in FIG. 1 is constructed in entirely the same way asthe conventional solid-state image pickup device shown in FIG. 24.

In the upper layer portion of an n-type substrate 5, a p-layer 6 havingdepth and density in given ranges is formed. A photoelectric transducer,an n-layer 7, is formed in a given region in the interior of the p-layer6. A p-layer 8 with a high density is also formed in the p-layer 6 at aposition spaced apart from the n-layer 7. An n-layer 9 that serves as avertical CCD transfer channel is provided in the p-layer 8. Signalcharges accumulated in the n-layer 7 are read out to the n-layer 9 andthen the signal charges are transferred within the n-layer 9 and in thedirection perpendicular to the paper face of the drawing. A high-densityp-layer 10 is formed on the n-layer 7 and in the surface of thesubstrate 5. The p-layer 10 is formed so that any dark current caused byan interfacial level of Si-SiO₂ can be prevented from occurring.

An insulating film 12 comprising SiO₂ or the like is formed on thesurface of the substrate 5. On the insulating film 12, a vertical CCDtransfer electrode 13 is formed above the p-layer 8 and n-layer 9 thatconstitute the vertical CCD transfer electrode area 2 and also above theregion embracing a gap region between the n-layer 7 and the p-layer 8.The insulating film 12 is also formed on the side wall and upper surfaceof the transfer electrode 13 for the purpose of protecting thesolid-state image pickup device from a physical impact or shock.

The vertical CCD transfer electrode 13 acts also as an electrode formaking the electrons accumulated in the n-layer 7 read out to thep-layer 9 via the p-layer 8 that forms the vertical CCD transferchannel.

FIG. 2 shows impurity density distribution examined along the line 2--2passing across the p-layer 10, photoelectric transducer n-layer 7,p-layer 6 and substrate 5a shown in FIG. 1.

FIG. 3 shows the net impurity distribution of the impurity densitydistribution shown in FIG. 2.

In the present invention, as shown in FIG. 1, the cross section of thesolid-state image pickup device has the same form as that of theconventional device, but has a different impurity density distributionof the impurity layers formed. This will be more detailed below.

In both FIGS. 2 and 3, the impurity densities are plotted as ordinate,and the distance from the surface of the substrate 5 is plotted asabscissa. The densities of p-type impurities are indicated above theabscissa. i.e., on the upper side of the ordinate, and the densities ofn-type impurities are indicated below the abscissa, i.e., on the lowerside of the ordinate.

Broken lines 21, 22, 23 and 24 in FIG. 2 represent the impuritydensities of the p-layer 6, substrate 5a, n-layer 7 and p-layer 10 inFIG. 1, respectively.

A solid line 25 in FIG. 3 represents the value obtained by synthesizingthe respective impurity densities shown in FIG. 2 (which is hereinafterreferred to as the net value). The shaded region 26 corresponds to thenet impurity density of the photoelectric transducer n-layer 7, showingthe amount of effective donors accumulated in the n-layer.

As to the impurity density 23 of the photoelectric transducer n-layer 7,a point at which the density is highest, i.e., the density is maximum,is present in the interior of the substrate 5. At the surface of thesubstrate 5, the impurity density of the n-layer 7 is low and thedensity increases with a depth toward the interior of the substrate 5.Thus, a maximum value of the impurity density of the n-layer 7 ispresent in the interior of the substrate 5, and the impurity density ofthe n-layer 7 decreases with increasing depth toward the interior of thesubstrate beyond the depth at which the density shows the maximum value.In the conventional solid-state image pickup device, the device is soformed to have the region in which the impurity density of the n-layer 7is maximum at the surface of the substrate 5 or in the region of littledepth corresponding to a depth at which the impurity density of thep-layer 10 for decreasing dark currents is distributed.

The p-layer 10 is formed at the upper part of the n-layer 7 so that thedark current can be decreased. The impurity density 24 of the p-layer 10is higher than the density of the n-layer 7, where the layer is soformed that the impurity density is only a little spread toward theinterior of the substrate 5.

The p-layer 6 is formed in the interior of the substrate 5 and in such away that it overlaps at least with an end portion of the photoelectrictransducer n-layer 7, located in the depth of the interior of thesubstrate 5. Its impurity density 21 begins to become higher from thepart at which the p-layer 6 and the p-layer 7 overlap one another, andalso is so distributed as to have a maximum value at the depth of thesubstrate 5. In the conventional solid-state image pickup device, thedevice is so formed to have the region in which the impurity density ofthe p-layer 6 is maximum at the surface of the substrate 5 or in theregion of little depth corresponding to a depth at which the impuritydensity of the p-layer 10 for decreasing dark currents is distributed.Moreover, its impurity density is distributed in the state that itreaches a fairly great depth of the substrate 5 in a low density.

The difference in the state of distribution of impurity density betweenthe solid-state image pickup device of the present invention and theconventional one brings about a great difference in the amount ofeffective donors accumulated in the photoelectric transducer.

More specifically, the amount of effective donors that is represented bythe net impurity density 26 can be obtained in the following way: It canbe obtained by subtracting the impurity density 24 of the p-layer 10 andthe impurity density 21 of the p-layer 6 from a value obtained by addingthe impurity density 22 of the substrate 5a to the impurity density 23.

In the conventional solid-state image pickup device, the device is soformed to have the region in which the impurity density of the n-layer 7is maximum at the surface of the substrate 5 or in the region of littledepth corresponding to a depth at which the impurity density of thep-layer 10 for decreasing dark currents is distributed. As a result, theregion having a highest impurity density within the n-layer 7 serving asthe photoelectric transducer is cancelled by the p-layer 10 formed atthe surface of the substrate 5 and having the state of distribution oflittle depth. This results in a lowering of the absolute amount ofeffective doners accumulated in the photoelectric transducer. Moreover,since the impurity density of the p-layer 6 is distributed deeply,though in a low density, to the depth of the substrate 5, the impuritydensity of the n-layer 7 becomes lower as a whole. This also results ina lowering of the absolute amount of effective donors accumulated in thephotoelectric transducer.

On the other hand, in the solid-state image pickup device of the presentinvention, the photoelectric transducer n-layer 7 is so formed as tohave an extremely low impurity density at the surface of the substrate 5or in the region of little depth corresponding to a depth at which theimpurity density of the p-layer 10 for decreasing dark currents isdistributed. Hence, the impurity density that may be cancelled in then-layer 7 serving as the photoelectric transducer, by the impuritydensity 24 formed in a high density at the surface of the substrate 5,that is, the p-layer 10 formed at the surface of the substrate 5 andhaving the state of distribution of little depth, is on an insignificantlevel. Thus the absolute amount of effective donors accumulated in aninsignificant photoelectric transducer may only be lowered to the extentcompared with the case of the conventional device.

In addition, as to the impurity density 21 of the p-layer 6, theimpurity density 21 is formed in the depth of the substrate 5. An end ofthe impurity density 21 is so distributed as to overlap with an end ofthe photoelectric transducer n-layer 7. At this part also, the densityof n-type impurities in the region of the impurity density 23overlapping one another with the impurity density 21 is so formed as tobe extremely low. Hence, the impurity density that may be cancelled inthe n-layer 7 serving as the photoelectric transducer, by the impuritydensity 21 is on an insignificant level. Thus, the absolute amount ofeffective donors accumulated in the photoelectric transducer may only belowered to extent compared with the case of the conventional device.When the p-layer 6 and n-layer 7 are overlapped in this way, theimpurity density 21 is subject to the following limitations.

In the first place, when there is no overlap, the solid line 25 of thenet impurity density shown in FIG. 3 comes into the shape as shown inFIG. 4. A region is produced such that the region in which the effectivedonors are accumulated protrudes to the depth of the substrate 5 in theform of a projection. In the photoelectric transducer having such ashape, it can not occur that the net impurity density 26 is cancelled bythe impurity density 21, so that a substantial net impurity densitybecomes larger. As the net impurity density 26 becomes larger, theamount of effective donors becomes larger.

Secondly, when the overlapping region is of little depth, the density ina maximum value a of the impurity density 21 in the overlapping regionmust be larger than impurity density b of the substrate 5a. In otherwords, the density a indicates the impurity density of the p-layer 6 atan end portion where the impurity density of the n-layer 7 isdistributed. If the value a is smaller than the value b, the impuritydensity at the depth of the substrate 5 comes into the form of aprojection like the solid line 25 of the net impurity density shown inFIG. 4.

If, on the other hand, the value a is extremely larger than the value b,a wall corresponding to the rise of the solid line 25 on the p-sidebecomes higher. In other words, the potential barrier formed between then-layer 7 and the substrate 5a becomes higher. A high potential barriermakes it possible prevent the blooming phenomenon by increasing thevoltage to be applied to the substrate 5.

When, however, the distribution of the impurity density 21 is so sharpthat the region in which the n-layer 7 and the p-layer 6 overlap oneanother is of little depth, the value a is extremely larger than thevalue b and also the p-layer 6 has a smaller thickness the net impuritydensity 26 may be cancelled by the impurity density 25 in a smallerquantity, so that the net impurity density 26 becomes higher. Thisresults in a larger amount of effective donors. A satisfactory effectcan also be obtained even when a low voltage is applied to the substrate5.

The impurity density 23 of the n-layer 7 has its maximum point in theregion in which the impurity densities 21 and 24 of the p-layers 6 and10, respectively, do not overlap at all. Forming the impurity density 23in this way makes it possible to increase the amount of effectivedonors.

Thus, the impurity density 23 of the n-layer 7 overlaps one another withthe impurity densities 21 and 24 of the p-layers 6 and 10, respectively,only at their end portions. In other words, the impurity density of thep-layer 10 and the impurity density 21 of the p-layer 6 do not overlapone another and both are continuous through the impurity density 23 ofthe n-layer 7. Formation of such impurity density distributions makes itpossible to remarkably increase the amount of effective donorsaccumulated in the photoelectric transducer n-layer 7.

On the other hand, in the net impurity density 25, the area of theshaded portion 26 that indicates the amount of effective donorsaccumulated in the photoelectric transducer n-layer 7 is larger than thearea of the shaded portion 19 (FIG. 27) that indicates the amount in theconventional case.

Hence, the upper limit value of saturation characteristics of thephotoelectric transducer can be made larger than that of theconventional one, and the quantity of saturation charges of thesolid-state image pickup device, i.e., dynamic range, can be greatlyimproved.

Here, the substrate 5a has an impurity density of about 10¹⁵ cm⁻³. Thep-layer 10 has an impurity density of 10¹⁸ cm⁻³ to 10¹⁹ cm⁻³ and has adiffusion length of 0.5 micron. The p-layer 6 has an impurity density ofabout 10¹⁵ cm⁻³ and its region starts at a depth of 1.5 microns from thesurface of the substrate 5, having a diffusion length of 3.0 microns.The n-layer 7 has an impurity density of 10¹⁶ cm⁻³ to 10¹⁷ cm⁻³ with itsdiffusion length of 1.8 microns. Therefore the region in which then-layer 7 and the p-layer 6 overlap one another is 0.3 micron.

FIG. 5 shows impurity densities in a solid-state image pickup deviceaccording to a second embodiment.

FIG. 5 shows impurity densities in the substrate, along the line 2--2passing across the n-layer 7, p-layer 6 and substrate 5a shown in FIG.1.

FIG. 6 shows net values of the impurity densities shown in FIG. 5.

In the present invention, as shown in FIG. 1, the cross section of thesolid-state image pickup device has the same form as that of theconventional device, but has a different impurity density distributionof the impurity layers formed. This will be more detailed below.

In both FIGS. 5 and 6, the ordinate and abscissa represent the same asthose in FIGS. 2 and 3.

Broken lines 27, 28, 29 and 30 in FIG. 5 represent the impuritydensities of the p-layer 6, substrate 5a, n-layer 7 and p-layer 10 inFIG. 1, respectively.

A solid line 31 in FIG. 6 represents the net value of each impuritydensity shown in FIG. 2. The shaded region 32 corresponds to the netimpurity density of the photoelectric transducer n-layer 7, showing theamount of effective donors accumulated in the n-layer.

As to the impurity density 29 of the photoelectric transducer n-layer 7,a point at which the density is highest. i.e.. the density is maximum,is present in the interior of the substrate 5. At the surface of thesubstrate 5, the impurity density of the n-layer 7 is low and thedensity increases with a depth toward the interior of the substrate 5.Thus, a maximum value of the impurity density of the n-layer 7 ispresent in the interior of the substrate 5, and the impurity density ofthe n-layer 7 decreases with increasing depth toward the interior of thesubstrate beyond the depth at which the density shows the maximum value.Thus, the distribution of the impurity density of the n-layer 7 is thesame as that in the first embodiment.

The p-layer 10 is formed at the upper part of the n-layer 7 so that thedark current can be decreased. The impurity density 30 of the p-layer 10is higher than the density of the n-layer 7, where the layer is soformed that the impurity density is only a little spread toward theinterior of the substrate 5. This is also the same as in the firstembodiment.

What is different from the first embodiment is that the p-layer 6 isformed in a low density distributed from the surface of the substrate 5to the depth of the substrate 5, in the same way as the impurity density15 in the conventional solid-state image pickup device.

More specifically, the impurity density 27 of the p-layer 6 has amaximum value at the surface of the substrate 5. It is distributed inthe state that the impurity density spreads to the depth of thesubstrate 5. The p-layer 6 is formed to have a diffusion depth which isgreater than the diffusion depth of the n-layer 7.

The difference in the state of distribution of impurity density betweenthe solid-state image pickup device of the present invention and theconventional one brings about a great difference in the amount ofeffective donors accumulated in the photoelectric transducer.

More specifically, the amount of effective donors that is represented bythe net impurity density 32 can be obtained in the following way: It canbe obtained by subtracting the impurity density 30 of the p-layer 10 andthe impurity density 27 of the p-layer 6 from a value obtained by addingthe impurity density 28 of the substrate 5a to the impurity density 29.

In the conventional solid-state image pickup device, the device is soformed to have the region in which the impurity density of the n-layer 7is maximum at the surface of the substrate 5 or in the region of littledepth corresponding to a depth at which the impurity density of thep-layer 10 for decreasing dark currents is distributed. As a result, theregion having a highest impurity density within the n-layer 7 serving asthe photoelectric transducer is cancelled by the p-layer 10 formed atthe surface of the substrate 5 and having the state of distribution oflittle depth. This results in a lowering of the absolute amount ofeffective doners accumulated in the photoelectric transducer. Moreover,since the impurity density of the p-layer 6 is distributed deeply,though in a low density, to the depth of the substrate 5, the impuritydensity of the n-layer 7 becomes lower as a whole. This also results ina lowering of the absolute amount of effective donors accumulated in thephotoelectric transducer.

On the other hand, in the solid-state image pickup device of the presentinvention, the photoelectric transducer n-layer 7 is so formed as tohave an extremely low impurity density at the surface of the substrate 5or in the region of little depth corresponding to a depth at which theimpurity density of the p-layer 10 for decreasing dark currents isdistributed. Hence, the impurity density that may be cancelled in then-layer 7 serving as the photoelectric transducer, by the impuritydensity 30 formed in a high density at the surface of the substrate 5,that is, the p-layer 10 formed at the surface of the substrate 5 andhaving the state of distribution of little depth, is on an insignificantlevel. Thus the absolute amount of effective donors accumulated in aninsignificant photoelectric transducer may only be lowered to the extentcompared with the case of the conventional device.

In the second embodiment, however, the impurity density 27 of thep-layer 6 is distributed from the surface of the substrate 5 to thedepth of the substrate 5. In the first embodiment, the impurity density21, corresponding to the impurity density 27, of the p-layer 6 is formedin the depth of the substrate 5. An end of the impurity density 21corresponding to the impurity density 27 is so distributed as to overlapwith an end of the photoelectric transducer n-layer 7. Hence, theimpurity density that may be cancelled in the n-layer 7 serving as thephotoelectric transducer, by the impurity density 21 corresponding tothe impurity density 27 is on the level almost out of question, aspreviously stated.

Compared with this, the second embodiment is the same as theconventional solid state image pickup device in that the p-layer 6 isformed with the impurity density diffused from the surface of thesubstrate 5. Hence the absolute amount of effective donors accumulatedin the photoelectric transducer becomes smaller than that in the firstembodiment. However compared with the conventional device, the amount ofeffective donors may only be lowered to an insignificant extent also inthe second embodiment.

In the first embodiment, the impurity density 21 corresponding to theimpurity density 27 is subject to the stated limitations when thep-layer 6 and n-layer 7 are overlapped. In the second embodiment,however, they can be formed with ease using a conventional process.

In the net impurity density 31, the area of the shaded portion 26 thatindicates the amount of effective donors accumulated in thephotoelectric transducer n-layer 7 is larger than the area of the shadedportion 19 (FIG. 27) that indicates the amount in the conventional case.

Hence, the upper limit value of saturation characteristics of thephotoelectric transducer can be made larger than that of theconventional one, and the quantity of saturation charges of thesolid-state image pickup device can be greatly improved.

Here, the substrate 5a has an impurity density of about 10¹⁵ cm⁻³. Thep-layer 10 has an impurity density of 10¹⁸ cm⁻³ to 10¹⁹ cm⁻³ and has adiffusion length of 0.5 micron. The p-layer 6 has an impurity density ofabout 2×10¹⁴ cm⁻³ and its region has a diffusion length of 5.5 micronsfrom the surface of the substrate 5. The n-layer 7 has an impuritydensity of 10¹⁶ cm⁻³ to 10¹⁷ cm⁻³ with its diffusion length of 1.8microns. Therefore the region in which the n-layer 7 and the p-layer 6overlap one another is 1.8 micron.

Fabrication of solid-state image pickup devices under conditions asdescribed above can achieve a saturation charge quantity of about1.2×10⁵ per picture element, which has been about 4×10⁴ per pictureelement in the conventional solid-state image pickup device.

FIG. 7 cross-sectionally illustrates a solid-state image pickup deviceaccording to a third embodiment of the present invention.

FIG. 7 is a cross-section of the device having the regions correspondingto those of the solid-state image pickup device shown in the prior artFIG. 25.

In the upper layer portion of an n-type substrate 5, a p-layer 33 havingdepth and density in given ranges is formed. A region corresponding topart of an n-layer photoelectric transducer 7 is formed in the p-layer33. A p-layer 8 with a high density is also formed in the p-layer 33. Ann-layer 9 that serves as a vertical CCD transfer channel is provided inthe p-layer 8. A p-layer 34 with a high density and a small thickness isfurther formed in contact with the bottom of the photoelectrictransducer 7. The p-layer 34 is formed in the n-type substrate 5 at itspart positioned between the p-layer 33 and another p-layer 33 adjacentthereto. A high-density p-layer 10 is formed on the photoelectrictransducer 7. The p-layer 8 is formed apart from the n-layer 7 andp-layer 10.

On the n-type substrate 5, a vertical CCD transfer electrode 13 isformed in the region from which a given region of the photoelectrictransducer 7 at least is extruded, interposing an insulating film 12comprising SiO₂ or the like.

The photoelectric transducer n-layer 7 is formed on the two regionscorresponding to the p-layer 33 with a low density and a large thicknessand the p-layer 34 with a high density and a small thickness.

Signal charges accumulated in the n-layer 7 are read out to the n-layer9 and then the signal charges are transferred within the n-layer 9 andin the direction perpendicular to the paper face of the drawing.

The high-density p-layer 10 formed on the photoelectric transducern-layer 7 is formed so that any dark current caused by an interfaciallevel of Si-SiO₂ can be prevented from occurring.

In the solid-state image pickup device of the present invention asdescribed above, what is different from the conventional solid-stateimage pickup device is that the photoelectric transducer n-layer 7 isformed at least in contact with the region extending over the tworegions corresponding to the p-layer 33 with a low density and a largethickness and the p-layer 34 with a high density and a small thickness.

Here, the substrate 5a used has an impurity density of 10¹⁵ cm⁻³. Thep-layer 33 has an impurity density of 10¹⁵ cm⁻³ with its diffusionlength of 3 microns. The n-layer 7 has an impurity density of 10¹⁶ cm⁻³to 10¹⁷ cm⁻³ with its diffusion length of 1.8 microns. The p-layer 10has an impurity density of 10¹⁸ cm⁻³ to 10¹⁹ cm⁻³ with its diffusionlength of 0.5 micron. The p-layer 8 has an impurity density of about3×10¹⁷ cm⁻³ with its diffusion length of 1.2 microns. The n-layer 9 hasan impurity density of 5×10¹⁶ cm⁻³ with its diffusion length of 0.8micron. The p-layer 34 has an impurity density of 4×10¹⁵ cm⁻³ with itsdiffusion length of 1.5 microns.

The p-layer 34 is formed at the position of the n-layer 7 serving as thephotoelectric transducer, spaced apart from the n-layer 9 serving as thevertical CCD transfer area. The p-layer 34 is formed along a long sideof the photoelectric transducer n-layer 7. The p-layer 34 is 3 micronsin length extending in the long-side direction. When it is formed in afurther extended length in this length along the long side of thephotoelectric transducer n-layer 7, it is possible to lower the voltageapplied for operating an electronic shutter. In other words, it ispossible to lower the pulse voltage to be applied to the substrate 5 soas for the signal charges accumulated in the n-layer 7 to be allreleased to the substrate 5a.

In the present embodiment, end portions of the photoelectric transducern-layer 7, and the p-layer 34 which are most distant from the verticalCCD transfer area, n-layer, designated as 9 in the drawing, are on thesame line. The same effect can be obtained also when the p-layer 34 isformed in such a way that its end portion extends from the end portionof the n-layer 7 to an adjoining p-layer 33 located in the left side inthe drawing. Thus, the positional relationship has an allowance in theformation of the end portions and hence a greater margin can be promisedin the manufacture.

On the other hand, if the end portion of the p-layer 34 is recessed fromthe end portion of the n-layer 7, it follows that the n-layer 7 comes indirect contact with the substrate 5a and therefore no signal charges canbe accumulated in the n-layer 7.

The p-layer 34 has a thickness of 1.5 microns in the depth direction ofthe substrate 5. This thickness influences the values of voltagesapplied to the n-layer 7 when the electronic shutter of the solid-stateimage pickup device is operated. More specifically, when the n-layer hasa thickness smaller than this thickness, the potential barrier producedbetween the n-layer 7 and the substrate 5a becomes lower. Hence thevoltage to be applied to the substrate 5 can be decreased.

When on the other hand the n-layer 7 is formed in a thickness largerthan this thickness, the potential barrier produced between the n-layer7 and the substrate 5a becomes higher. Hence, the voltage to be appliedto the substrate 5 must be increased.

A reverse bias voltage Vsub that causes positive potential in the n-typesubstrate 5a and negative potential in the p-layer 33 is applied acrossthe p-layer 33 and the n-type substrate 5a.

FIGS. 8 and 9 show potential distributions along the line 8--8 and 9--9,respectively, shown in FIG. 7, which are formed when a positive voltageVsub is applied to the n-type substrate 5 in order to prevent theblooming phenomenon.

FIG. 8 is a graph to explain in detail that the solid-state image pickupdevice of the present invention is effective for decreasing the bloomingphenomenon.

In FIG. 8, a broken line 36 represents the potential distribution alongthe line 8--8 shown in FIG. 7, and a solid line 37 the potentialdistribution along the line 9--9. Regions 10, 7, 33 and 5 indicatedepth-direction thicknesses of the high-density p-layer 10, thephotoelectric transducer n-layer 7, the thick, low-density p-layer 33and the n-type substrate 5a, respectively.

When the Vsub potential has been applied, the value of minimum potential38 in the thick, low-density p-layer 33 is higher than the value ofminimum potential 39 of the thin, high-density p-layer 34.

In the photoelectric transducer n-layer 7, a potential well is formed.Hence, the charges produced in the n-layer 7 and its circumference movealong the potential that forms this potential well. As a result, chargesare accumulated in the potential well. With a further increase in thecharges accumulated to bring about charges having the potential thatexceeds the minimum potential 38, the charges become excess and movealong the potential distribution of the p-layer 33. As a result, theexcess charges are released to the n-type substrate 5a.

The blooming phenomenon is a phenomenon that occurs as a result ofincidence of highly luminous light on the photoelectric transducern-layer 7. More specifically, the light made incident instantaneouslyproduces a large quantity of electrons in the photoelectric transducer.Only a given volume of electrons can be accumulated in the photoelectrictransducer. When the electrons thus instantaneously produced are in alarge quantity and are more than the volume within which they areaccumulated in the photoelectric transducer, the blooming phenomenon iscaused by excess electrons flowing into the adjacent vertical CCDtransfer channel. Such a phenomenon is called blooming.

As is seen from FIG. 8, the potential distribution in the photoelectrictransducer is indicated by two types of distribution corresponding tothe broken line 36 and solid line 37 shown in FIG. 8. The electronsformed in the photoelectric transducer n-layer 7 are accumulated in thepotential well. Having been accumulated in a large quantity, theelectrons flow to the p-layer 33 from the part of n-layer 7, havingpotential distribution of the lower potential in the two distributionforms of the photoelectric transducer n-layer 7. The minimum potential38 in the state of distribution as shown by the solid line 37 is lowerthan the minimum potential 39 of the broken line 36, and hence theelectrons flow to the p-layer 33 firstly from the region having thepotential distribution shown by the solid line 37.

FIG. 9 is a graph to explain in detail that the solid-state image pickupdevice of the present invention is effective for achieving-the operationof an electronic shutter.

The operation of an electronic shutter is to release all the signalcharges accumulated in the photoelectric transducer n-layer 7 to then-type substrate 5a. FIG. 9 shows potential distribution in thesubstrate 5a, examined when the electronic shutter was operated. Inother words, the operation of the electronic shutter is to release allthe electrons accumulated in the photoelectric transducer n-layer 7 tothe substrate 5a. Unless the accumulated electrons are completelyreleased, a state in which charges in a quantity different for eachphotoelectric transducer unit are accumulated (or left) in the interioris brought about when the electronic shutter is operated. As a result,fixed patterns are produced because of the charges left therein.

In order to operate the electronic shutter, a positive voltage Vsubwhich is much higher than the case of FIG. 8 is applied to the n-typesubstrate 5.

In FIG. 9, a broken line 40 and a solid line 41 represent the potentialdistribution along the line 8--8 and line 9--9, respectively, shown inFIG. 7.

Regions 10, 7, 33 and 5 in FIG. 9 indicate depth-direction thicknessesof the high density p-layer 10, the photoelectric transducer n-layer 7,the thick, low-density p-layer 33 and the n-type substrate 5a,respectively.

When the Vsub potential bas been applied, the minimum potential 42 inthe thin, high-density p-layer 34 is higher than the minimum potential43 of the thick, low-density p-layer 33, and also the minimum potential42 is lower than the potential 44 of the n-layer 7. Hence, the signalcharges accumulated in the photoelectric transducer n-layer 7 move alongthe potential represented by the broken line 40 indicating a lowerpotential. At this time, the potential represented by the broken line islower than the minimum potential 44 in the photoelectric transducern-layer 7, and hence all the signal charges accumulated in thephotoelectric transducer are released to the n-type substrate 5a.

FIG. 10 is a graph to show the relationship between Vsub and potentialto explain the reason for the above occurrence.

In FIG. 10, a solid line 45 and a broken line 46 each show the Vsubvoltage dependence of minimum potentials in the thick, low-densityp-layer 33 and the thin, high density p-layer 34, respectively.

The solid line 45 indicates that the minimum potential in the p-layer 33gradually increases with an increase in the Vsub potential.

As for the broken line 46, it indicates that the minimum potential inthe p=layer 34 abruptly increases with an increase in the Vsubpotential.

This difference results from the fact that, in comparison between thep-layer 33 and the p-layer 34, the impurity density of the p-layer 33 islower than the impurity density of the p-layer 34 and also the formerhas a larger thickness than the latter. In other words, the p-layer 34has on the other hand a higher impurity density and is formed in asmaller thickness.

Hence, the Vsub potential dependence of the quantity of chargesaccumulated in the photoelectric transducer n-layer 7 is determined bythe solid line 45 when the minimum potential is below the intersection47 and by the broken line 46 when it is above the intersection 47. Morespecifically, the quantity of charges accumulated in the photoelectrictransducer has a Vsub potential dependence as shown in FIG. 11. As isseen therefrom, a while during which the charges are accumulated in thephotoelectric transducer, i.e., the state as shown in FIG. 8 correspondsto a region 48 shown in FIG. 11.

When signal charges are released by operating the electronic shutter, itis operated in the state as shown in FIG. g, i.e.. the state in which ahigher voltage than that of a solid line shown in FIG. 11 is applied. Bydoing so, the photoelectric transducer having a high saturated chargequantity can be operated as the electronic shutter at a low Vsub pulsevoltage.

Thus, in order to prevent occurrence of the blooming, the photoelectrictransducer should preferably be operated in a region like the region 48in which the changes in maximum quantity of accumulated charges have amild Vsub potential dependence with respect to the Vsub potential. Usethereof in such a region enables easy control of Vsub potential ofcamera-combined VTRs. Use thereof in such a Vsub potential region alsomakes it possible to lessen the decrease in the maximum quantity ofcharges accumulated in photoelectric transducers.

On the other hand, when the electronic shutter is operated, thephotoelectric transducer should preferably be used in a region like theregion 49 in which the changes in maximum quantity of accumulatedcharges have a sharp Vsub potential dependence with respect to the Vsubpotential.

This is because the potential at which the electronic shutter isoperated is attained by further adding a clock voltage to the potentialapplied when the blooming is prevented from occurring.

In the solid-state image pickup device of the present invention, theelectronic shutter can be operated at a voltage lower than the clockvoltage applied when an electronic shutter is operated in theconventional solid-state image pickup device.

More specifically, when the electronic shutter is operated in theconventional solid-state image pickup device, the clock voltage appliedis 30 V. On the other hand, in the solid-state image pickup deviceaccording to the third embodiment, the electronic shutter can beoperated at a voltage as low as 18 V.

Thus, the thick, low-density p-layer 33 and the thin, high densityp-layer 34 may be formed in contact with the photoelectric transducern-layer 7, so that the Vsub potential of a video camera for preventingthe blooming can be controlled with ease and also the maximum quantityof charges accumulated in the photoelectric transducer (i.e., saturationcharge quantity) can be increased. The operation of the electronicshutter can also be achieved at a low clock voltage.

The p-layer 34 with a high-density and a small thickness may be furtherformed in such a way that its part overlaps in the photoelectrictransducer n-layer 7 as shown by a broken line 35 in FIG. 7. That is tosay, its formation at the position as shown by the broken line 35 bringsabout a substantial reduction of the thickness of the n-layer 7 cominginto contact with the thin, high-density p-layer 34. Hence, when thesignal charges of the photoelectric transducer are read out, thephotoelectric transducer n-layer 7 is brought into a depleted state. Asa result, the potential distribution produced there becomes deeper onthe side of read-out and hence the occurrence of residual images can beprevented with greater ease.

Now, regarding the solid-state image pickup device shown in FIG. 7, itsdriving method will be described with reference to FIGS. 8 and 9.

In the device shown in FIG. 7, the p-layer 8 is electrically grounded. Apositive voltage is applied to the substrate 5. Hence, the interfacebetween the p-layers 8, 33 and 34 and the substrate 5a is brought intothe state of a reverse bias. As previously described, FIG. 8 is thegraph in which the potential distribution in the period during whichsignal charges are accumulated at this time is shown as potentialdistribution along the lines 8--8 and 9--9 in FIG. 7.

In this period, the minimum potential 38 of the p-layer 33 is deeperthan the minimum potential 39 of the p-layer 34. Hence the electronsthat have become excess in the photoelectric transducer n-layer 7 mustmove along the minimum potential 38 and be released to the substrate 5a.For this purpose, a given voltage may be applied to the substrate 5.

On the other hand, FIG. 9 is the graph in which the potentialdistribution in the period during which the electronic shutter isoperated is shown as potential distribution along the lines 8--8 and9--9 in FIG. 7.

In this period, a voltage much higher than that in the period duringwhich signal charges are accumulated must be applied to the substrate 5.Application of much higher voltage to the substrate 5 makes it possiblefor all the signal charges accumulated in the n-layer 7 to be releasedto the n-type substrate 5a. For this purpose, the minimum potential 42of the p-layer 34 must be set to have a higher potential than themaximum potential 44 of the n-layer 7.

The p-layer 33 may be made to have a low density and be large in itsthickness of distribution and also the p-layer 34 may be formed in ahigh density to the depth, so that the electronic shutter can beoperated at a voltage lower than the conventional case. At this time,the potential of the minimum potential 43 of the p-layer 33 is lowerthan that of the minimum potential 42 of the p-layer 34.

In other words, the minimum potential 43 of the p-layer 33 can surpassthe minimum potential 42 of the p-layer 34 with an increase in the Vsubpotential applied to the substrate 5.

Thus, the electronic-shutter can be operated even with application of alow potential to the substrate 5.

A fourth embodiment of the solid-state image pickup device according tothe present invention will be described below with reference to FIGS. 12and 13.

FIG. 12 is a cross section along the line 25--25 of the prior art device(FIG. 24).

In the upper layer portion of an n-type substrate 5, a p-layer 33 havingdepth and density in given ranges is formed. A region corresponding topart of an n-layer photoelectric transducer 7 is formed in the p-layer33. A p-layer 8 with a high density is also formed in the p-layer 33. Ann-layer 9 that serves as a vertical CCD transfer channel is provided inthe p-layer 8. A p-layer 34 with a high density and a small thickness isfurther formed in contact with part of the bottom of the photoelectrictransducer 7. The p-layer 34 is formed in the n-type substrate 5 at itspart positioned between the p-layer 33 and another p-layer 33 adjacentthereto. A high-density p-layer 10 is formed on the photoelectrictransducer 7. The p-layer 8 is formed apart from the n-layer 7 andp-layer 10.

On the n-type substrate 5, a vertical CCD transfer electrode 13 isformed in the region from which a given region of the photoelectrictransducer 7 at least is extruded, interposing an insulating film 12comprising SiO₂ or the like.

In this embodiment, what is different from the first embodiment is thatthe p-layer 34 with a high density and a small thickness is formed onthe read-out side of the p-layer 8 serving as a vertical CCD transferchannel.

More specifically, the part coming into contact with the thin,high-density p-layer 34 within the photoelectric transducer n-layer 7 isformed in a depth a little larger than the part coming into contact withthe p-layer 33 having a low density and a large thickness. Thephotoelectric transducer n-layer 7 has a shape of a hook, at the end ofthe shorter side of which the p-layer 34 is formed. This constitutionbrings about an increase in potential on the read-out side of thephotoelectric transducer when the photoelectric transducer n-layer 7 hasbeen depleted. Hence, the charges in the photoelectric transducer canmove with ease at the time of read-out. It therefore becomes easier toprevent residual images from occurring.

FIG. 13 is a cross section along the line 13--13 of the prior art device(FIG. 24).

In the upper layer portion of the n-type substrate 5, a p-layer 33having depth and density in given ranges is formed. A region positionedbetween the p-layer 33 and its adjacent p-layer 33 defines part of then-type substrate 5. A p-layer 34 is formed in the n-type substrate 5 atthe position held between the p-layer 33 and its adjacent p-layer 33. Ann-layer 7 serving as a photoelectric transducer is continuously formedon each partial region of the p-layer 33 and p-layer 34. A separatingp-layer 52 is formed between the photoelectric transducers adjacent oneanother.

In comparison of the present invention of this embodiment with the firstand second embodiments, the p-layer 8 formed in the p-layer 33 and then-layer 9 serving as a transfer channel formed in the p-layer 8 are notprovided in this embodiment.

The p-layer 34 with a high density and a small thickness is formed incontact with part of the bottom of the photoelectric transducer 7. Ahigh-density p-layer 10 is formed on the photoelectric transducern-layer 7.

On the n-type substrate 5, a vertical CCD transfer electrode 50 isformed in the region from which a given region of the photoelectrictransducer 7 at least is extruded, interposing an insulating film 12comprising SiO₂ or the like. Another vertical CCD transfer electrode 51is also fromed interposing the insulating film 12.

In regard to the p-layer 34 with a high density and a small thickness,one layer is formed at the separating part between a photoelectrictransducer and its adjacent photoelectric transducer. In other words,the p-layer 34 is formed in common to two photoelectric transducers.

With this constitution, the number for the formation of the thin,high-density p-layer 34 ca be reduced to the half of the number ofphotoelectric transducers. The thin, high-density p-layer 34 coming intocontact with the photoelectric transducer n-layer 7 can also be made tohave a smaller area, and hence it becomes easy to prevent occurrence ofthe blooming.

FIG. 14 shows density distribution of impurity atoms present in thethickness direction along the lines 14--14 and 15--15 in FIG. 12. Thenumerals 53, 54, 55, 56 and 57 denote impurity densities of thehigh-density p-layer 10, the photoelectric transducer n-layer 7, thethin, high-density p-layer 34, the thick, low-density p-layer 33 and then-type substrate 5a, respectively.

In order to form the p-layer 34 to give a layer with a high density anda small thickness, having the impurity density 55, the impurity density54 of the photoelectric transducer n-layer 7 is so formed as to have amaximum value of impurity atom density in a more interior part from thesurface of the substrate 5. With such structure, it becomes possible tolessen the decrease in net impurity density in the photoelectrictransducer n-layer 7. Hence, the characteristics of the photoelectrictransducer can not be deteriorated. At this time, the impurity density56 of the thick, low-density p-layer 33 may also be made to have thestructure that it has a maximum value of impurity atom density in a moreinterior part from the surface of the substrate 5 as in the case of thethin, high-density p-layer 34. With such structure, the independencebetween the impurity density 56 of the thick, low-density p-layer 33 andthe impurity density 55 of the thin, high-density p-layer 34 can be keptwith ease to make it easier to control impurity densities.

Formation of the structure as described above can be achieved bycarrying out ion implantation of impurities such as boron at anaccelerating energy of 200 keV or more when the thin, high-densityp-layer 34 is formed.

The net impurity density distribution in the circumference of thephotoelectric transducer thus formed is as shown in FIG. 15. Thenumerals 58, 59, 60 and 61 denote net impurity densities of thehigh-density p-layer 10, the photoelectric transducer n-layer 7, thethin, high-density p-layer 34 and the n-type substrate 5a, respectively.The numerals 62 and 63 denote net impurity densities of the thick,low-density p-layer 33 and the n-type substrate 5a, respectively. Atthis time, the thick, low-density p-layer 33 may also be formed by ionimplantation at an accelerating energy of 200 keV or more as in the caseof the thin, high-density p-layer 34, thereby making it easier tocontrol impurity densities.

FIGS. 16 to 19 cross-sectionally illustrate a flow sheet for fabricatingthe solid-state image pickup device according to the first embodiment.

In the main surface of the substrate 5 with a specific resistance of 20Ω.cm, p-type impurity boron is ion-implanted to cover substantially thewhole surface. Here, the boron is ion-implanted in an amount of about5×10¹¹ /cm². As a result of this ion implantation, boron is led into thesurface of the substrate 5.

Thereafter, the substrate 5 is subjected to a heat treatment to form thep-layer 6.

In the solid-state image pickup device according to the secondembodiment previously described, ions are led into the surface of thesubstrate 5 and then thermally diffused so that its impurity density canbe distributed over a wide range. More specifically, ion-implanted boronis diffused by a high-temperature heat treatment to form the p-layer 6.

The p-layer 6 is so formed that its impurity density can be distributedover a wide range within the region of the p-layer 6, since ions are ledinto the substrate surface and then ions must be diffused over a widerange by a high-temperature heat-treatment.

Thereafter, using conventional photolithography, a resist pattern isformed on the region other than the region that serves for the verticalCCD transfer channel. Using the resist pattern as a mask, boron ision-implanted to form the p-layer 8.

Thereafter, the above resist pattern is removed by dry etching using anoxygen type gas. Using again photolithography, a resist pattern is alsoformed on the region other than the region in which the n-layer 9 withinthe p-layer 8 serving as the vertical CCD transfer channel is formed.

Using the resist pattern as a mask, phosphorus is ion-implanted to formthe n-layer 9. Thereafter, the resist pattern is removed by oxygen-typedry etching. Next, the insulating film 12 is formed on the main surfaceof the substrate 5 by thermal oxidation. Here, an oxide film is used asthe insulating film 12. On the insulating film 12, an electrode materialserving as the vertical CCD transfer electrode 13 is further formed. Aresist 20' for a resit pattern is further coated on the electrodematerial to form a resist pattern 20 (FIG. 16).

Next, the resist on the region other than the region broader than theregion serving for the transfer electrode 13, i.e., on the region inwhich the n-layer 7 serving as the photoelectric transducer is formed,is removed by conventional photolithography to form a resist pattern 20.Using the resist pattern 20 as a mask, the electrode material isdry-etched until the insulating film 12 is uncovered.

Next, using the resist pattern 20 as a mask and also making theinsulating film 12 serve as a protective film, phosphorus ision-implanted. The n-layer 7 serving as the photoelectric transducer isformed as a result of this ion implantation (FIGS. 17 to 18). Here, theion implantation of phosphorus is carried out at an accelerating energyof 360 keV to 800 keV and in an amount of about 2×10¹² /cm² to 3.4×10¹²/cm².

As is evident from the above, it is unnecessary to increase the quantityof the impurities led into the photoelectric transducer, which has beendone in the prior art. To increase the quantity of impuritiesnecessarily brings about a considerable loss of time. Thus, in theprocess of the present invention, there can be no lowering of throughputat the time of the fabrication of solid-state image pickup devices.

It is also unnecessary to diffuse the implanted impurities into thedepth of the substrate by a high-temperature heat treatment so that thearea of the photoelectric transducer can be increased. Hence, no defectscan occur in the substrate, which may be caused by the high temperatureheat treatment. Since the impurities are diffused from otherdiffusion-layer, it is still also unnecessary to make control so as toattain the desired impurity density. There also is no possibility thatthe photoelectric transducer is diffused and spread as a result of thehigh-temperature heat treatment and consequently diffused also to thevertical CCD transfer channel or the gap region between the transferchannel and the photoelectric transducer electrode 13.

As for the positional relationship between the end portion of thephotoelectric transducer and the end portion of the vertical CCDtransfer electrode, it can not become difficult to control the positionof the diffusion layer as a result of the high-temperature heattreatment. Hence the positional relationship between the both can beattained with ease. This can prevent the blooming phenomenon fromoccurring in the solid-state image pickup device, and also can stop thedeterioration of picture quality that may be caused by a lowering ofsaturation characteristics.

Thereafter, the resist pattern 20 is removed. On the surface of thesubstrate 5, an electrode material is formed with a region broader thanthe insulating film 12 and transfer electrode 13. Next, a resist patternis formed on the region other than the region in which the transferelectrode 13 is formed. Using this resist pattern as a mask, theelectrode material is dry-etched. Through the above process, thetransfer electrode 13 is formed (FIG. 18).

At this time, the transfer electrode 13 must be provided also on the gapregion between the n-layer 7, i.e., the photoelectric transducer fromwhich electrons are read out to the vertical CCD transfer channel, andthe p-layer 8. For this purpose, since in the dry etching carried outwhen the transfer electrode 13 is formed a side wall end of the n-layer7 serving as the photoelectric transducer is originally on the same linewith a side wall end of the transfer electrode 13, one side wall end ofthe transfer electrode 13 is shortened to be formed in the desiredlength.

Next, using this transfer electrode 13 and a resist as masks, boron ision-implanted. The p-layer 10 for preventing dark current is thus formedon the n-layer 7 (FIG. 19).

An example of n-type photoelectric transducers has been described here,but of course the same effect can be obtained also in the case of p-typephotoelectric transducers.

The same effect can be obtained of course also when the polarity of theconductivity type is set reverse and the polarity of applied voltage isset reverse.

Also as a matter of course, implanting ion species are by no meanslimited to those set out herein.

FIGS. 20 to 23 cross-sectionally illustrate a flow sheet for fabricatingthe solid-state image pickup device according to the third embodimentpreviously described.

On the main surface of the n-type substrate 5, a resist pattern isformed on its region other than the region that serves for the p-layer.More specifically, the resist pattern is formed on the substrate 5corresponding to the region of the thin, high-density p-layer 34 formedbeneath the photoelectric transducer. Next, using the resist pattern asa mask, p-type impurity boron is ion-implanted to cover substantiallythe whole surface of the substrate 5.

Thereafter, the resist pattern is removed by oxygen-type dry etching.

Next, a resist pattern 64 is afresh formed on the region serving for thep-layer 33. More specifically, the resist pattern 64 is formed on thesubstrate 5 except its region for the thin, high-density p-layer 34formed beneath the photoelectric transducer. Next, using this resistpattern 64 as a mask, p-type impurity boron is ion-implanted into thesubstrate 6 (FIG. 20).

Thereafter, the resist pattern 64 is removed by oxygen-type dry etching.

In the above two steps of boron ion implantation, the order of theimplantation may be exchanged.

At this time, if the end portions of the diffusion layers of p-layer 33and p-layer 34 overlap one another, an increase in the impurity densityat the overlapping part and also an increase in thickness at that partmay be brought about. Hence the overlapping part is not contributory tothe releasing of signal charges to the substrate.

If on the other hand the diffusion-layers of p-layer 33 and p-layer 34are formed without overlap of their end portions, the density of p-typeimpurities at the non-overlapping gap portion becomes lower. Thisresults in a lowering of the saturation charge quantity or a lowering ofthe Vsub voltage to be applied.

Thereafter, a resist pattern is formed by photolithography on the regionother than the region serving for the vertical CCD transfer channel.Using the resist pattern as a mask, boron is ion-planted to form thep-layer 8.

Thereafter, the resist pattern is removed by dry etching using anoxygen-type gas. Using again photolithography, a resist pattern is alsoformed on the region other than the region in which the n-layer 9 withinthe p-layer 8 serving as the vertical CCD transfer channel is formed.

Using the resist pattern as a mask, phosphorus is ion-implanted to formthe n-layer 9.

Thereafter, the resist pattern is removed by oxygen-type dry etching.

Next, the insulating film 12 is formed on the main surface of thesubstrate 5 by thermal oxidation. Here, an oxide film is used as theinsulating film 12.

The thickness of this oxide film must be controlled in a high precisionsince the oxide film is used as a mask when the photoelectric transduceris formed by ion implantation in the subsequent step.

On the insulating film 12, an electrode material serving as the verticalCCD transfer electrode 13 is further formed.

A resist pattern 65 is further formed on the region other than theregion broader than the region serving for the transfer electrode 13.Using the resist pattern 65 as a mask, the electrode material isdry-etched until the insulating film 12 is uncovered (FIG. 21).

Next, using the resist pattern 65 as a mask and also making theinsulating film 12 serve as a protective film, phosphorus ision-implanted. The n-layer 7 serving as the photoelectric transducer isformed as a result of this ion implantation (FIG. 22).

The p-layer 7 has impurity density distributed in such a way that thedensity is high in the region formed in the substrate 5 and is low inthe region formed in the p-layer 33. Namely, the impurity density of thephotoelectric transducer n-layer 7 formed in the substrate is determinedby the sum of the density of n-type impurities of the substrate 5 andthe density of ion-implanted n-type impurities. On the other hand, as tothe photoelectric transducer n-layer 7 formed in the p-layer 33, itsimpurity density is determined by the sum of the density of n-typeimpurities of the p-layer 33 and the density of ion-implanted n-typeimpurities.

In this way, regions having different impurity densities are formed inone photoelectric transducer n-layer 7.

The thin, high-density p-layer 34 is so controlled as to be formed atthe bottom of the diffusion-layer of n-layer 7 serving as thephotoelectric transducer.

In the fourth embodiment of the solid-state image pickup device aspreviously described, the thin, high-density p-layer 34 is formed on theside of the transfer electrode 13.

In such an instance also, the region in which the p-layer 34 and then-layer 7 overlap one another has impurity density corresponding to thesum of the impurity density of the n-layer 7 and that of the impuritydensity of the p-layer.

Here, the photoelectric transducer n-layer 7 is formed in such a waythat its part coming into contact with the thin, high-density p-layer 34is formed in a depth a little larger than its part coming into contactwith the thick, low-density p-layer 33. The photoelectric transducern-layer 7 has a shape of a hook, at the end of the shorter side of whichthe p-layer 34 is formed. This constitution brings about an increase inpotential on the read-out side of the photoelectric transducer when thephotoelectric transducer n-layer 7 has been depleted. Hence, it becomespossible for the charges in the photoelectric transducer to move withease at the time of read-out. It therefore becomes easier to preventresidual images from occurring.

In this way, the hook-shaped n-layer is formed in such a way that thejoint surface between the p-layer 34 and the n-layer 7 is deeper thanthe joint surface between the p-layer 33 and the n-layer 7. Morespecifically, when the p-layer 34 is formed, the ion implantation ofboron is carried out at a higher accelerating energy than theaccelerating energy used in the ion implantation carried out when thep-layer 33 is formed. Thus, the thickness of the n-layer 7 in the depthdirection of the substrate 5 can be increased, so that the potential atthat part can be made higher.

As is evident from the above, it is unnecessary to increase the quantityof the impurities led into the photoelectric transducer, which has beendone in the prior art. To increase the quantity of impuritiesnecessarily brings about a considerable loss of time. Thus, in theprocess of the present invention, there can be no lowering of throughputat the time of the fabrication of solid-state image pickup devices.

It is also unnecessary to diffuse the implanted impurities into thedepth of the substrate by a high-temperature heat treatment so that thearea of the photoelectric transducer can be increased. Hence, no defectscan occur in the substrate, which may be caused by the high temperatureheat treatment. Since the impurities are diffused from otherdiffusion-layer, it is still also unnecessary to make control so as toattain the desired impurity density. There also is no possibility thatthe photoelectric transducer is diffused and spread as a result of thehigh-temperature heat treatment and consequently diffused also to thevertical CCD transfer channel or the gap region between the transferchannel and the photoelectric transducer electrode 13.

As for the positional relationship between the end portion of thephotoelectric transducer and the end portion of the vertical CCDtransfer electrode, it can not become difficult to control the positionof the diffusion layer as a result of the high-temperature heattreatment. Hence the positional relationship between both can beattained with ease. This can prevent the blooming phenomenon fromoccurring in the solid-state image pickup device, and also can stop thedeterioration of picture quality.

Thereafter, the resist pattern 65 is removed. On the surface of thesubstrate 5, an electrode material is formed with a region broader thanthe insulating film 12 and transfer electrode 13. Next, a resist patternis formed on the region other than the region in which the transferelectrode 13 is formed. Using this resist pattern as a mask, theelectrode material is dry-etched. Through the above process, thetransfer electrode 13 is formed (FIG. 22).

At this time, the transfer electrode 13 must be provided also on the gapregion between the n-layer 7, i.e., the photoelectric transducer fromwhich electrons are read out to the vertical CCD transfer channel, andthe p-layer 8. For this purpose, since in the dry etching carried outwhen the transfer electrode 13 is formed a side wall end of the n-layer7 serving as the photoelectric transducer is originally on the same linewith a side wall end of the transfer electrode 13, one side wall end ofthe transfer electrode 13 is shortened to be formed in the desiredlength.

Next, using this transfer electrode 13 and a resist as masks, boron ision-implanted. The p-layer 10 for preventing dark current is thus formedon the n-layer 7 (FIG. 23).

As described above, in the process for manufacturing the solid-stateimage pickup device according to the present invention, the n-layer thatserves as a photoelectric transducer is formed by ion implantation usinga high accelerating energy, and hence the impurity density distributioncan be made to have a highest density region in the interior of thesubstrate.

This brings about an increase in the amount of the effective doners thatcan be accumulated in the photoelectric transducer, and achieves thequantity of saturation charges at a sufficiently high level.

Since the photoelectric transducer is formed by ion implantation, thepositional relationship between the end portion of the photoelectrictransducer and the vertical CCD transfer electrode can be determined bythe precision for the registration of a mask in the step of exposurecarried out when the transfer electrode 13 is formed. Hence it isunnecessary to control the position at which the photoelectrictransducer is formed by heat diffusion at a high temperature. Thus, ahigh controllability can be achieved for the positions of the endportion of the photoelectric transducer and the vertical CCD transferelectrode. This makes the solid-state image pickup device hardly causethe blooming phenomenon, and makes it possible to prevent deteriorationof picture quality.

In the impurity density distribution of the n-layer in which thephotoelectric transducer is formed by ion implantation, it is possiblefor the maximum value of its density to be present in the interior ofthe substrate. This brings about an increase in the amount of theeffective doners accumulated in the photoelectric transducer. Hence, itis possible to obtain a solid-state image pickup device equipped with aphotoelectric transducer having high saturation characteristics.

The p-layer for preventing dark currents is also formed on thephotoelectric transducer n-layer. The n-layer serving as thephotoelectric transducer is formed by ion implantation to have a maximumvalue in its impurity density in the interior of the substrate. Hence,the net value of the impurity density of the p-layer at the surface ofthe substrate can be made to have a higher density than the impuritydensity of the p-layer of the conventional solid-state image pickupdevice. This makes it possible to obtain a solid-state image pickupdevice equipped with a photoelectric transducer having low dark currentcharacteristics.

In the process for fabricating the solid-state image pickup device ofthe present invention, the impurity density of the photoelectrictransducer is highest in the depth of the substrate. Hence, it isunnecessary to increase the quantity of the impurities implanted in thephotoelectric transducer, which has been done in the prior art. Toincrease the quantity of impurities necessarily brings about aconsiderable loss of time. Thus, in the process of the presentinvention, there can be no lowering of throughput at the time of thefabrication of solid-state image pickup devices. In addition, it ispossible to prevent the yield from being lowered because of the whitescratches.

It is also unnecessary to diffuse the implanted impurities into thedepth of the substrate by a high-temperature heat treatment so that thearea of the photoelectric transducer can be increased. Hence, no defectscan occur in the substrate, which may be caused by the high-temperatureheat treatment. Since the impurities are diffused from otherdiffusion-layer, it is still also unnecessary to make control so as toattain the desired impurity density. There also is no possibility thatthe photoelectric transducer is diffused and spread as a result of thehigh-temperature heat treatment and consequently diffused also to thevertical CCD transfer channel or the gap region between the transferchannel and the photoelectric transducer electrode 13.

As for the positional relationship between the end portion of thephotoelectric transducer and the end portion of the vertical CCDtransfer electrode, it can not become difficult to control the positionof the diffusion layer as a result of the high-temperature heattreatment. Hence the positional relationship between the two can beattained with ease. This can prevent the phenomena of blooming andresidual image from occurring in the solid-state image pickup device,and also can restrain the deterioration of picture quality.

Examples of n-type photoelectric transducers have been described here,but of course the same effect can be obtained also in the case of p-typephotoelectric transducers.

What is claimed is:
 1. A solid-state image pickup device comprising asemiconductor substrate; a first semiconductor region of oneconductivity type, provided in said semiconductor substrate; a secondsemiconductor region of a conductivity type reverse to said firstsemiconductor region, formed in said first semiconductor region; a thirdsemiconductor region of the same conductivity type as said firstsemiconductor region, formed adjoiningly to the surface of said secondsemiconductor region; and a fourth semiconductor region to constitute aCCD transfer channel of the same conductivity type as said secondsemiconductor region, formed apart from said second semiconductor regionwithin said first semiconductor region;said image pickup device having anet atomic impurity density distribution as a synthesis of individualatomic impurity densities of said first, second and third semiconductorregions, said first, second and third semiconductor regions havingrespective impurity density distributions as components of said netimpurity density distribution; said second semiconductor region havingan individual atomic impurity density whose distribution in the depthdirection of said semiconductor substrate has a maximum at a portioninterior from the surface of said semiconductor substrate.
 2. Asolid-state image pickup device comprising a semiconductor substrate; afirst semiconductor region of one conductivity type, provided in saidsemiconductor substrate; a second semiconductor region of a conductivitytype reverse to said first semiconductor region, formed in said firstsemiconductor region; a third semiconductor region of the sameconductivity type as said first semiconductor region, formed adjoininglyto the surface of said second semiconductor region; and a fourthsemiconductor region to constitute a CCD transfer channel of the sameconductivity type as said second semiconductor region, formed apart fromsaid second semiconductor region within said first semiconductorregion;said image pickup device having a net atomic impurity densitydistribution as a synthesis of individual atomic impurity densities ofsaid first, second and third semiconductor regions, said first, secondand third semiconductor regions having respective impurity densitydistributions as components of said net impurity density distribution;said first semiconductor region and said second semiconductor regioneach having a respective individual atomic impurity density whosedistribution in the depth direction of said semiconductor substrate hasa maximum at a portion interior from the surface of said semiconductorsubstrate.
 3. A solid-state image pickup device comprising asemiconductor substrate; a plurality of first semiconductor regions ofone conductivity type, formed in given regions of said semiconductorsubstrate; a second semiconductor region of a conductivity type reverseto said first semiconductor regions, formed in one of said firstsemiconductor regions and in a region between said one firstsemiconductor region and another first semiconductor region adjacentthereto; a third semiconductor region of the same conductivity type assaid first semiconductor region, formed at least on the surface of saidone first semiconductor region; a fourth semiconductor region toconstitute a CCD transfer channel of the same conductivity type as saidsecond semiconductor region, formed apart from said second semiconductorregion within said one first semiconductor region; and a fifthsemiconductor region of a conductivity type reverse to saidsemiconductor substrate, formed between said one first semiconductorregion and said another first semiconductor region adjacent thereto. 4.A method of driving a solid-state image pickup device, said devicecomprising a semiconductor substrate; a plurality of first semiconductorregions of one conductivity type, formed in a given region of saidsemiconductor substrate; a second semiconductor region of a conductivitytype reverse to said first semiconductor region, formed in one saidfirst semiconductor region and in a region between said firstsemiconductor region and another first semiconductor region adjacentthereto; a third semiconductor region of the same conductivity type assaid first semiconductor region, formed at least on the surface of saidfirst semiconductor region; a fourth semiconductor region to constitutea CCD transfer channel of the same conductivity type as said secondsemiconductor region, formed apart from said second semiconductor regionwithin said one first semiconductor region; and a fifth semiconductorregion of a conductivity type reverse to said second semiconductorregion, formed between said one first semiconductor region and saidanother first semiconductor region adjacent thereto;said methodcomprising applying a reverse bias voltage across said firstsemiconductor regions and said semiconductor substrate in such a waythat a period during which said reverse bias voltage is applied iscomprised of a first application period and a second application period,the minimum potential of said first semiconductor regions during thefirst application period is higher than the minimum potential of saidfifth semiconductor region, and the minimum potential of said fifthsemiconductor region during the second application period is higher thanthe minimum potential of said first semiconductor region.
 5. Thesolid-state image pickup device according to claim 3, wherein said fifthsemiconductor region is adjoiningly formed to said second semiconductorregion on a side opposite to a region where the second semiconductorregion is disposed against said fourth semiconductor region in the firstsemiconductor region, said first semiconductor region being of the sameconductivity type as said fifth semiconductor region.
 6. The solid-stateimage pickup device according to claim 3, wherein the minimum potentialof said first semiconductor region is higher than the minimum potentialof said fifth semiconductor region.
 7. The solid-state image pickupdevice according to claim 3, wherein said fifth semiconductor region isformed adjoiningly to at least a bottom part of said secondsemiconductor region.
 8. A solid-state image pickup device according toclaim 7, wherein said fifth semiconductor region is formed as one of incontact with part of, and overlapping with, part of said secondsemiconductor region.
 9. A solid-state image pickup device according toclaim 7, wherein the position of the bottom of said fifth semiconductorregion in the depth direction of said semiconductor substrate is setshallower than the position of the bottom of said first semiconductorregion in the depth direction of said semiconductor substrate.
 10. Asolid-state image pickup device according to claim 7, wherein thedistribution of impurity density of said second semiconductor region inthe depth direction of said semiconductor substrate has a maximum partof an interior part from the surface of said semiconductor substrate.11. A solid-state image pickup device according to claim 7, wherein thedistribution of impurity density of each of said first semiconductorregion and said second semiconductor region in the depth direction ofsaid semiconductor substrate has a maximum part of an interior part fromthe surface of said semiconductor substrate.
 12. A solid-state imagepickup device as recited in claim 1, wherein the individual atomicimpurity density of the second semiconductor region is equal to anindividual atomic impurity density of the semiconductor substrate at twodifferent distances from the surface of the semiconductor substrate inthe interior portion of the semiconductor substrate.
 13. A solid-stateimage pickup device as recited in claim 2, wherein the individual atomicimpurity density of the second semiconductor region is equal to anindividual atomic impurity density of the semiconductor substrate at twodifferent distances from the surface of the semiconductor substrate inthe interior portion of the semiconductor substrate; andthe individualatomic impurity density of the first semiconductor region is equal tothe individual atomic impurity density of the semiconductor substrate attwo different distances from the surface of the semiconductor substratein the interior portion of the semiconductor substrate.
 14. Thesolid-state image pickup device according to claim 12, wherein theindividual atomic impurity density of the second semiconductor region isequal to the individual atomic impurity density of the semiconductorsubstrate at first and second locations in the interior portion of thesemiconductor substrate, wherein a nearer one of said first and secondlocations to the surface of the semiconductor substrate is in the thirdsemiconductor region.
 15. The solid-state image pickup device accordingto claim 14, further including first, second and third positions in theinterior portion of the semiconductor substrate, said first, second andthird positions having successively greater distances from the surfaceof said semiconductor substrate, respectively, wherein:said firstposition is at said nearer location wherein said individual atomicimpurity density of said second semiconductor region equals theindividual atomic impurity density of said semiconductor substrate; saidsecond position is at a location where an individual atomic impuritydensity of the third semiconductor region is equal to the individualatomic impurity density of the semiconductor substrate; and said thirdposition is at a location where the individual atomic impurity densityof the second semiconductor region is a maximum in the depth directionof the semiconductor substrate.
 16. The solid-state image pickup deviceaccording to claim 12, wherein said net atomic impurity densitydistribution is formed by a sum of the individual atomic impuritydensity distributions of the semiconductor substrate and the first,second and third semiconductor regions,said net atomic impurity densitydistribution having at least a part where the individual atomic impuritydensity distribution of the second semiconductor region is a maximum inthe depth direction of the semiconductor substrate within a region wheredonors are accumulated.
 17. The solid-state image pickup deviceaccording to claim 13, wherein there exists within the secondsemiconductor region a position where the individual atomic impuritydensity of the third semiconductor region is equal to the individualatomic impurity density of the semiconductor substrate; andwherein thereexist two positions wherein the individual atomic impurity density ofthe second semiconductor region is equal to the individual atomicimpurity density of the semiconductor substrate; a closer one of saidtwo positions to the surface of the semiconductor substrate being withinthe third semiconductor region.
 18. The solid-state image pickup deviceaccording to claim 13, wherein the individual atomic impurity density ofthe first semiconductor region is equal to the individual atomicimpurity density of the semiconductor substrate at two locations andamong these two locations, the location closer to the surface of thesemiconductor substrate exists in the second semiconductor region; andthe individual atomic impurity density of the second semiconductorregion is equal to the individual atomic impurity density of thesemiconductor substrate at two positions and among these two positions,the position being farther removed from the surface of the semiconductorsubstrate exists in the first semiconductor region.
 19. The solid-stateimage pickup device according to claim 13, wherein a position where theindividual atomic impurity density of the second semiconductor region ismaximum in the depth direction of the semiconductor substrate, existsbetween:a position where the individual atomic impurity density of thethird semiconductor region is equal to the individual atomic impuritydensity of the semiconductor substrate; and one of two locations wherethe individual atomic impurity density of the first semiconductor regionis equal to the individual atomic impurity density of the semiconductorsubstrate, the one location being the closer of the two locations to thesurface of the semiconductor substrate.
 20. The solid-state image pickupdevice according to claim 13, wherein said net atomic impurity densitydistribution is formed by a sum of the individual atomic impuritydensity distributions of the semiconductor substrate and the first,second and third semiconductor regions,said net atomic impurity densitydistribution having at least a part where the individual atomic impuritydensity distribution of the second semiconductor region is a maximum inthe depth direction of the semiconductor substrate within a region wheredonors are accumulated.